Hah - calibre strikes again.
It is a font issue. The problem lies in the book's css:
Code:
.calibre {
display: block;
font-family: "DejaVu", sans;
font-size: 1em;
margin-bottom: 0;
margin-left: 5pt;
margin-right: 5pt;
margin-top: 0;
page-break-before: always;
text-align: justify
}
Remove the line saying
font-family: "DejaVu", sans;
since this is forcing your reader to use its default sans-serif font rather than the new font, which was defined as the default serif. If you want a font that's a bit more readable you could try
Gentium, though it doesn't have a bold weight.